1. Field of the Invention
This invention relates generally to the field of X-ray diffraction and, more specifically, to the centering of a sample crystal for single-crystal diffraction analysis.
2. Description of the Related Art
Single-crystal X-ray diffraction (SC-XRD) is a method for determining the three-dimensional atomic structure of a crystalline compound. A single-crystal specimen of the compound is irradiated with monochromatic X-ray radiation from different directions, some of which is diffracted in specific patterns and detected by an active pixel sensor. The structural information of the specimen is determined from the geometry and relative intensities of these diffraction patterns. The intensities are integrated from the pixels in the active pixel array sensor images.
A typical laboratory system 100 for performing single-crystal diffraction experiments normally consists of five components as shown in FIG. 1. The components include an X-ray source 102 that produces a primary X-ray beam 104 with the required radiation energy, focal spot size and intensity. X-ray optics 106 are provided to condition the primary X-ray beam 104 to a conditioned, or incident, beam 108 with the required wavelength, beam focus size, beam profile and divergence. A goniometer 110 is used to establish and manipulate geometric relationships between the incident X-ray beam 108, the crystal sample 112 and the X-ray sensor 114. The incident X-ray beam 108 strikes the crystal sample 112 and produces scattered X-rays 116 which are recorded in the sensor 114. A sample alignment and monitor assembly comprises a sample illuminator 118 that illuminates the sample 112 and a sample monitor 120, typically a video camera, which generates a video image of the sample to assist users in positioning the sample in the instrument center and monitoring the sample state and position.
The goniometer 110 allows the crystal sample 112 to be rotated around several axes. Precise crystallography requires that the sample crystal 112 be aligned to the center of the goniometer 110 and maintained in that center when rotated around the goniometer rotational axes during data collection. During exposure, the sample (a single crystal of the compound of interest) is rotated in the X-ray beam 108 through a precise angular range with a precise angular velocity. The purpose of this rotation is to predictably bring Bragg reflections into constructive interference with the incident beam 108. During this time, called the charge integration time, the pixels of the sensor receive and integrate the X-ray signals.
The centering of the crystal in the goniometer prior to the start of the experiment may be done by a human operator by manually adjusting the translations of a goniometer head or by indicating the location of the specimen on a computer screen. Alternatively, the crystal centering can be done automatically using one of a number of conventional centering methods. These prior art methods fall, generally, into three different categories: mechanical methods; optical methods using computer vision algorithms; and X-ray diffraction based methods that use the intensity (or lack thereof) of the diffraction pattern produced by the specimen.
In the past, optical methods of centering appear to have been the most commonly used although, when automated, there are often problems with finding small crystals or those located in liquids. X-ray diffraction based methods of crystal centering have relied on the movement of the crystal relative to the X-ray beam while collecting diffraction images. Typically, the area to be scanned is broken down into points in a grid, and the X-ray beam is used to illuminate each point in the grid. The detected signal is then checked for the presence of diffraction spots for each of the illuminated grid points. In this way, the edges of the crystal can be found and, from that, the center of the crystal can be determined.